May 7 (Reuters) - Britain's main stock index fell to a one-month low on Tuesday as investors returned from a long weekend to a threat by U.S. President Donald Trump to China on tariffs.

The FTSE 100 was down 0.2 percent by 0816 GMT after falling as much as 0.7 percent. Midcaps were 0.1 percent lower as security group G4S weighed on the index after Canada's Garda World abandoned its takeover plan.

Trump's threat to impose additional tariffs on Chinese goods hit Asia-exposed financial stocks. HSBC and Prudential were among the biggest drags on the blue-chip index.

Industrial groups Melrose and Ashtead, which are vulnerable to uncertainties in the global market, lost 2.5 percent and 1 percent respectively.

Investors steered away from riskier assets and moved to defensive stocks, which some consider to be safer in times of economic uncertainty. Pharmaceutical groups Hikma, AstraZeneca and utility SSE rose.

"There is a sense that the U.S. is working extremely hard to extract last-minute concessions from China ahead of a planned visit by Vice-Premier Liu He," Neil Wilson, an analyst at Markets.com, said.

Helping to control the losses were Rio Tinto and BHP , which rose after a gain in Chinese iron ore futures due to concerns over tight supply.

Vodafone added 1.4 percent after an agreement to give Telefonica Deutschland access to its high-speed broadband network to help secure approval for its Liberty Global deal.

The stock slumped 6.4 percent after the company said it no longer expected its international business to break even this year.

Security group G4S gave up 3 percent after Garda World said on Sunday it would not make an offer for the company, while Essentra skidded 5.2 percent after a rating downgrade by Stifel.

AIM-listed Purplebricks dropped 6 percent after the online estate agent said its founder and chief executive, Michael Bruce, would quit immediately as the company winds down its Australian operations and reviews its U.S. business. (Reporting by Muvija M in Bengaluru, editing by Larry King and Dale Hudson)